1. Field of the Invention
The present invention relates to a fuel supply system, especially a fuel supply system having no return pipe (hereinafter called "a returnless fuel type") for returning excess fuel in a fuel rail or the like to a fuel tank. The term "fuel rail" means a fuel distributing pipe for distributing high pressure fuel supplied by a fuel pump to fuel injection valves.
2. Description of Related Art
Conventionally, the generation of vapor in the fuel is suppressed by maintaining the fuel in a fuel rail or the like at high pressure, to enable engine restarting even after the stop of an internal combustion engine (hereinafter called "an engine"). For example, U.S. Pat. No. 5,361,742 disposes a check valve between a fuel pump and the fuel rail in parallel with a differential pressure valve to keep the fuel pressure in the fuel rail or the like high after the engine stops.
According to such a construction, however, the differential pressure valve is required besides the check valve, which increases the number of necessary parts as well as raises manufacturing costs.
Under such a situation, U.S. Pat. No. 5,339,785 uses a check valve as a valve member moving to the fuel upstream side or the downstream side due to a differential pressure between the upstream and the downstream sides of a differential pressure valve.
According to this returnless fuel system disclosed in U.S. Pat. No. 5,339,785, a fuel amount matching the amount injected by a fuel injection valve is supplied by a fuel pump controlled by an electronic control unit. This control system transits a discharge pressure control signal to the fuel pump for the electronic control unit based on fuel pressure inside a fuel rail.
However, since this fuel pump is controlled via the electronic control unit, an electric signal transmission delay occurs in addition to delays of fuel pump response performance pressure transfer inside fuel supply pipes. Thus, control of the fuel pump discharge pressure lags behind pressure changes inside the fuel rail caused by fuel injection. This causes a possible fuel pressure rise due to pressure imbalance inside the fuel rail, which operates the check valve unnecessarily. More specifically, when such a control delay exists fuel cut-off during rapid deceleration of a vehicle, fuel discharge pressure does not lower even through there is now a lower fuel injection requirement. This makes the downstream side of the check valve pressure higher than that of the upstream side. Consequently the check valve closes due to such a pressure difference. In this case, "a state of fuel cut-off" means that the injection fuel amount is small or zero and, as a result, the discharge of the fuel pump reduces accompanied by a reduction in fuel amount discharged by the fuel pump. Once the check valve operates, the fuel rail pressure is maintained at a high value and, even after the stare of fuel cut-off, the fuel rail pressure is maintained at a high value. Thus, fuel pressure becomes unnecessarily high and causes improper fuel injection as well as degradation of fuel efficiency.